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Date:   Mon, 4 Jan 2021 18:57:17 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        sargun@sargun.me, amir73il@gmail.com, vgoyal@redhat.com
Subject: Re: [PATCH][RESEND] vfs: serialize updates to file->f_sb_err with
 f_lock
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On Mon, Jan 04, 2021 at 01:43:47PM -0500, Jeff Layton wrote:
> @@ -172,7 +172,12 @@ SYSCALL_DEFINE1(syncfs, int, fd)
>  	ret = sync_filesystem(sb);
>  	up_read(&sb->s_umount);
>  
> -	ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
> +	if (errseq_check(&sb->s_wb_err, f.file->f_sb_err)) {
> +		/* Something changed, must use slow path */
> +		spin_lock(&f.file->f_lock);
> +		ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
> +		spin_unlock(&f.file->f_lock);
> +	}

	Is there any point bothering with the fastpath here?
I mean, look at the up_read() immediately prior to that thing...